Various types of implantable leads are known and used. Cardiac pulse generators, in particular, use implantable leads to both sense cardiac function and deliver stimulation pulses. One type of commonly used implantable lead is an endocardial lead.
Endocardial leads are attached at their proximal end to an implantable pulse generator and at their distal end to the endocardium (or even myocardium) of a cardiac chamber. Often the lead assembly is inserted into the heart through a vein. The lead generally has an inner conductor covered by an insulative sheath.
The distal end of an endocardial lead may electrically couple with the endocardium by either an active fixation mechanism or a passive fixation mechanism. Passive fixation mechanisms, such as a tine assembly, lodge or passively fix the lead to the heart. Active fixation mechanisms use a structure, such as a helix or hook, to engage into or actively fix themselves to the heart, and in particular, to the myocardium.
A sharpened helix has been found to provide a reasonably secure means for actively fixing a lead to the heart. One drawback to the use of a helix is the tissue reaction between the heart tissue and the rigid helix. Specifically, the implantation of the helix within the heart tissue triggers the foreign body response. That is, the body launches a series of attacks and processes upon the helix, intending to either destroy the helix or at least encapsulate it within a protective capsule of tissue. The most visible evidence of this foreign body response is the layer of activated macrophages and collagen capsule around the helical coil. Such tissues detrimentally affect the electrical performance of the surrounding tissue and thus the ability of the lead contacting that tissue to either electrically stimulate and sense the tissue. As a result, if the helix itself is used as the electrode, or even if the electrode is near the helix, the stimulation thresholds may rise, typically as a function of time after implant.
In particular, maintaining a low stimulation threshold for an implantable medical device is important. Implantable pulse generators are battery-powered and thus have a finite operating life. Over time, the battery will deplete and ultimately the implanted pulse generator must be replaced. Replacement, however, involves a surgical procedure and should be avoided, if possible. Therefore, it is important to minimize the electrical current drain on the battery. A lead which minimizes such drain by maintaining low stimulation thresholds is desired. One approach to maintain low stimulation thresholds is to prevent or control the build-up of tissues such as collagen around the fixation mechanism.